The long-term goal of our research is to understand the neural basis of plasticity of complex motivated behaviors. Towards this goal, we propose to investigate the circuits and neural mechanisms that are responsible for the generation of feeding behavior in a simple model system, Aplysia californica. This research will utilize an integrated interdisciplinary approach that combines anatomical, immunocytochemical, electrophysiological, and biochemical techniques with a behavioral analysis. The major question we will investigate concerns the ability of an organism to utilize the same neuronal elements and the same muscles in executing a variety of coordinated behaviors each of which can vary in their intensity and speed. We propose that this may be achieved by two major groups of mechanisms. The first, consists of creating different functional motor circuits, through activation of different combinations of motorneurons for different behaviors, and of changes of firing patterns of motorneurons in those cases when within a single behavior either the intensity or the frequency of responding change. The second postulated group of mechanisms operate by changing the proportions and combination of modulatory cotransmitters (especially neuropeptides) and classical transmitters acting on the muscle, thus adjusting the properties of the muscle to meet specific behavioral demands. A conceptual model that incorporates both groups of mechanisms will be tested, and if correct, will help guide future studies of the role that circuit selection and neuromodulation play in motivated behaviors. These studies should facilitate understanding of the neural basis of motivated behaviors and of their dysfunction, as seen in prevalent mental illnesses such as depression and mania.